1. Vocabulary
Hydrate
- chemically, a salt with water molecule(s) attached
MN. xH2O e.g. CaSO4 . ½ H2O
Anhydrous
- without water
MN. xH2O MN H2O D
Decrepitation
- water rapidly “popping” off a hydrate when heated rapidly
Hygroscopic
- absorbs moisture readily from its surroundings
The desiccator is used to store dried samples in a dry atmosphere. It should not be used to dry an object, but to maintain an already dried object indefinitely in a dry condition.
Most desiccants are ionic compounds that form hydrates. The most common used are CaCl2 (forms CaCl2 x 2 H2O) and CaSO4 (forms CaSO4 x 0.5 H2O). Silica gel (SiO2) is also used for some applications. Because desiccants absorb moisture, they should be handled with care, wearing goggles and a lab coat.
Desiccator image (top right) cat/categ01/sd200.asp
Gypsum (used in drywall) is CaSO4 . 2H2O (calcium sulfate dihydrate)
Turquoise (jewelry) is CuAl6(PO4)4(OH)8 . 4H2O (a tetrahydrate)
Desiccant
- a drying agent used to absorb moisture
Desiccator

2. Hydrate
Ionic compounds that contain specific ratios of loosely bound water molecules, called waters of hydration.
Waters of hydration can be removed by heating.
Compounds that differ only in the
numbers of waters of hydration
can have very different properties.
Copyright 2007 Pearson Benjamin Cummings. All rights reserved.

3. Step 1A) determine the mass of hydrate (MN. ? H2O)
Formula of a Hydrate
Given the following data
BaCl2.3H2O
mass of beaker  
47.28 g  
beaker
mass of beaker and sample before heating
53.84 g
beaker  +  MN  +  H2O
mass of beaker and sample after heating
51.84 g
beaker  +  MN
molar mass of anhydrous salt
128 g / mol A B C D
hydrate g MN. xH2O
Step 1A)  determine the mass of hydrate (MN. ? H2O)
                beaker  +  hydrate                g
        -   beaker                                  g
                MN. ? H2O  hydrate                 6.56 g

4. Formula of a Hydrate Given the following data
mass of beaker  
47.28 g  
beaker
mass of beaker and sample before heating
53.84 g
beaker  +  MN  +  H2O
mass of beaker and sample after heating
51.84 g
beaker  +  MN
molar mass of anhydrous salt
128 g / mol
hydrate g MN. xH2O
anhydrous salt g MN
Step 1B) determine the mass of anhydrous salt (MN)
                beaker  +  anhydrous salt     g
       -      beaker                                   g
                MN             anhydrous salt      4.56 g

5. Step 1C) determine the mass of water
Formula of a Hydrate
Given the following data
mass of beaker  
47.28 g  
beaker
mass of beaker and sample before heating
53.84 g
beaker  +  MN  +  H2O
mass of beaker and sample after heating
51.84 g
beaker  +  MN
molar mass of anhydrous salt
128 g / mol
hydrate g MN. xH2O
anhydrous salt g MN
Step 1C) determine the mass of water
                hydrate                                      6.56 g
       -      anhydrous salt                            4.56 g
                H2O           water                      g

6. Formula of a Hydrate Step 2A) convert grams to moles (salt)
                x mol MN  =  4.56 g MN (1 mol MN / 128 g/mol MN) 
                x  =  mol MN
Step 2B) convert grams to moles (water)
                x mol H2O  =  2.00 g H2O  (1 mol H2O / 18 g/mol H2O) 
                x  =  mol H2O
Step 3) divide by SMALLEST "# of moles“
                mol MN / mol  =  1 MN
                mol H2O / mol  =  3.12 H2O
To determine the empirical formula from the mass percentages of the elements in a compound, the following procedure is used:
1. The mass percentages are converted to relative numbers of atoms,
2. A 100 g sample of the compound, is assumed
3. Each of these masses is divided by the molar mass of the element to determine how many moles of each element are present in the 100
g sample
4. The results give ratios of the various elements in the sample—but whole numbers are needed for the empirical formula, which expresses the relative numbers of atoms in the smallest whole numbers possible
5. To obtain whole numbers, the number of moles of all the elements in the sample are divided by the number of moles of the element present in the lowest relative amount. Results will be the subscripts of the elements in the empirical formula
Step 4) use the ratio to find the hydrate's formula
                  1 MN H2O   (recall, it is not possible to have 3.12 molecules of water)
                   1 MN . 3H2O        Final Answer

7. Formula of a Hydrate - Prelab
Printable
copy of
LAB
1) Weigh dry beaker (record mass in data table)
2) Add 1 spoonful of hydrate to beaker (weigh)
3) HEAT: ~10 minutes
gently (low heat) ~3 minutes and HIGH ~7 minutes
4) Cool 3 minutes and weigh
5) HEAT: HIGH ~5 – 7 minutes
6) Cool 3 minutes and reweigh
7) Repeat steps #5 and #6 if “constant weight” is not observed
DATA TABLE
Molar mass of anhydrous salt = g
Percentage composition of water = 51.2%

8. Traditional heating of hydrate using a crucible.
Traditional heating of hydrate using a crucible.
I’ve had very good luck using a beaker.

9. Analysis
1. What is the mass difference between the hydrate and the ionic compound?
What does this mass represent?
2. To determine the formula of the hydrate, what piece of information do
you need?
Ask your teacher for this information.
3. What is the empirical formula of the hydrate?
Show your work.

10. Conclusions
1. Suppose the heating did not remove all of the water from the hydrate.
How would this affect your results?
2. List at least two other possible sources of error in this investigation.
you need?
Explore Further
A desiccant is a compound used to absorb moisture from the air.
How could the formula of a hydrate indicate its effectiveness as
a desiccant?